This is a Division of application Ser. No. 10/063,326 filed Apr. 11, 2002. The entire disclosure of the prior application is hereby incorporated by reference herein in its entirety.
This invention relates to reducing the porosity of porous thin films.
Substrates having thin films are used commonly in many applications. Thin films are deposited upon substrates used routinely, for instance, in integrated circuits. Likewise, substrates having thin films are used in a multitude of micro-electro-mechanical devices. Substrates having thin films are also used in the semiconductor industry, where great precision in the patterns formed in, and/or locations of, the thin film formed upon the substrates is necessary. The specific patterning and/or positioning of the thin film formed upon a substrate is typically achieved by controllably etching away the thin film material from the substrate using a photoresist mask, such that the thin film material remains in only those areas covered by the photoresist mask.
FIG. 1 shows steps corresponding to earlier methods of depositing thin films on substrates. The substrate may be any known or later-developed suitable material, such as glass, metal, or the like. In general, the substrate material will be appropriate for the application the thin film device will be used with. The thin film is formed by sputtering a layer of material used to form the thin film onto the substrate, applying a layer of photoresist materials over the sputtered thin film layer, masking and developing the photoresist material layer to a desired pattern, and then etching away the unwanted photoresist materials and portions of the thin film layer to achieve the desired pattern in the thin film. Often, the resulting thin film layer tends to be porous. That is, the thin film will have voids, pores, holes, cracks and/or other surface penetrating defects.
Porosity in sputtered thin films is a common occurrence. Such porosity in sputtered thin films contributes to problems, such as photoresist intrusions, that render subsequent photolithography difficult. Porosity in sputtered thin films also contributes to lateral etchant intrusions into sidewalls of a thin film, thus rendering subsequent thin film etching difficult with respect to the precision required in thin films. Likewise, porosity in sputtered thin films results in a decreased mechanical thin film strength. Because of the weakened nature of the thin film, wire-bonding becomes difficult or unreliable. Further, porosity in sputtered thin films results in a lack of lateral film cohesion, which unfavorably compromises the physical properties of the thin film. For example, the transverse electromechanical coupling co-efficient of a thin film may be compromised by the lack of lateral film cohesion that occurs in porous thin films.
After the thin film is deposited on the substrate, a photolithography/photoresist material layer is applied over the thin film layer. The photolithography/photoresist material layer is commonly formed by spin-casting the photolithography/photoresist material layer onto the thin film. Because the thin film has surface-penetrating defects, the photolithography/photoresist material penetrates into the porous thin film. The photolithography/photoresist material is then exposed through a mask to achieve a desired pattern of exposed/unexposed portions of the photolithography/photoresist material layer on the thin film.
Thereafter, the exposed, or unexposed, portion of the photolithography/photoresist material layer is washed away or otherwise removed to leave the desired pattern of photolithography/photoresist material layer on the thin film. An etchant to which the thin film, but not the photolithography/photoresist material layer, is sensitive is then applied to remove the portions of the thin film that are not protected by the photolithography/photoresist material layer. The remaining patterned photolithography/photoresist material layer is then removed, without damaging the underlying thin film to leave a patterned thin film layer.
While the above-described process is common, it also is inherently flawed in that the photoresist material, which is initially applied to the thin film and that penetrate the voids or pores of the porous thin film, is often not successfully completely removed in areas intended to be etched prior to etching the thin film. In this case, portions of the thin film that were not to be protected by the photolithography/photoresist material layer are not fully etched away by the etchant because some photoresist remains in the unintended areas. Alternatively, the voids or other surface penetrating defects can allow the etchant to penetrate under the patterned layer of the photolithography/photoresist material layer. In this case, portions of the thin film that were to be protected by the patterned layer of photolithography/photoresist material layer are nonetheless etched away. This results in the thin film pattern upon the substrate being not as accurate or precise as would be ideally provided by the patterned photolithography/photoresist material layer. Furthermore, any etchant that laterally intruded or migrated into unintended areas of the thin film may have weakened the thin film and substrate, and also results in a thin film pattern that is not as accurate or precise as desired.